Increase in the use of sophisticated car stereos in recent years has caused an increase in the number of thefts of the stereos in number. In order to prevent the number of thefts of the stereo, a number of antitheft systems have been proposed. In accordance with one of the systems, a detachable grille having a plurality of push buttons for control of the stereo is attached to the body of the stereo.
FIG. 5 shows the above described prior art. The car stereo comprises a car stereo body 1 and a detachable grille 20 attached to the body 1.
A front panel 11 having a recess 11a is fixed on the outer periphery of the front portion of the car stereo body 1. The recess 11a is so dimensioned that the grille 20 snugly fits therein. At an upper central portion of the recess 11a is formed a cassette opening 12. Three push buttons 13 are provided on the opposite sides of the cassette opening 12. A connector 14 which is connected to a connector 29 (FIG. 6) provided at the back of the grille 20 when the grille is attached, is formed on the panel 11. A volume control knob 15 is provided on the front panel 11 adjacent to the recess 11a.
The grille 20 has a cassette opening 21 which coincides with the cassette opening 12 of the body 1 and push buttons 22 provided on opposite sides of the cassette opening 21 corresponding to the push buttons 13 of the body 1 for operating a cassette tape. At a lower portion of the opening 21, six preset switches 23 are provided for setting a frequency of, for example, FM channels and for displaying one of the set frequencies. Function switches 24, time adjusting switches 25 and a display exchange switch 26 are provided at a lower portion of the preset switches 23. A liquid crystal display panel 27 is disposed on a right side of the grille 20 for displaying an operation mode of the car stereo.
In order to operate the car stereo, each of the stereo body 1 and the grille 20 has a control system as shown in FIG. 6. The control system for the grille 20 comprises a microcomputer 31, a key code producing circuit 33 connected with the operation switches 23 to 26 so as to transmit a key code in accordance with the operation of the switches, a display driver 30 operated by the microcomputer 31 for driving the liquid crystal display panel 27, a lighting lamp 32, and the connector 29. The control system for the stereo body 1 comprises a microcomputer 2 for controlling various operation modes of the car stereo, a 5V supply circuit 3 for supplying the voltage of 5V to the microcomputer 2, an 8V supply circuit 5, a protection circuit 4, and the connector 14.
The connector 14 of the body has a 5V supply terminal 14a, data output terminal 14b, data input terminal 14c, key code input terminal 14d and 8V supply terminal 14e. The terminals 14a and 14e are connected to the 5V supply circuit 3 and the 8V supply circuit 5, respectively, through the protection circuit 4. The data output and input terminals 14b and 14c are connected to the microcomputer 2. The microcomputer 2 is connected to the protection circuit 4 so as to operate the circuit. When the grille 20 is detached, the protection circuit 4 stops the supply of voltage from the 5V supply circuit 3 and 8V supply circuit 5 to the terminals 14a and 14e. Thus, the car stereo circuitry provided in the body 1 is protected from damage which is caused by the short-circuiting of the exposed terminals 14a to 14e.
The connector 29 of the grille 20 has terminals which correspond to the terminals 14a to 14e of the connector 14. Namely, there is provided a 5V input terminal 29a, data input terminal 29b, data output terminal 29c, key code output terminal 29d, and 8V input terminal 29e. The terminal 29a is connected to the microcomputer 31 so as to supply the 5V voltage thereto. The microcomputer 31 receives data from the microcomputer 2 through the data input and output terminals 29b and 14b dependent on the operation of the operation switches 23 to 26 on the grille 20. A signal of the microcomputer 31 is applied to the microcomputer 2 through data output terminal 29c and data input terminal 14c. The microcomputer 31 applies a control signal to the display driver 30 to indicate the received data on the display 27. The 8V input terminal 29e is connected to the lamp 32 for lighting the display 27.
When the grille 20 is taken out, since only the volume control knob 15 is provided on the stereo body 1, the stereo body loses the appearance as a car stereo. Hence, if the driver takes the grille 20 with him/her when leaving the vehicle, the theft of the car stereo is prevented.
In the conventional car stereo, the control system of the grille is complicated in construction and it is necessary to provide a large number of terminals in the connector. Thus, manufacturing cost of the car stereo is prevented from being reduced.
When the grille is detached from the body, the grille may be dropped accidentally. The liquid crystal material of the display is sealed between thin sheets of transparent electrodes forming a cell. Therefore if the display is depressed by shock, the cell may be deformed.
Furthermore, if the detached grille is placed near a heat source, the cell may also be deformed.